1. Field of the Invention
The present invention relates to scanned-stylus atomic force microscopes and a method of operating a scanned-stylus atomic force microscope, and in particular to large scan optical lever atomic force microscopes.
2. Discussion of the Background
Atomic force microscopes (AFMs) are extremely high resolution surface measuring instruments. The AFM is described in detail in several U.S. Patents including U.S. Pat. Nos. 4,935,634 to Hansma et al, 5,025,658 to Flings et al, and 5,144,833 to Amer et al. The AFM scans a stylus mounted on a flexible spring lever (cantilever) with respect to a sample. The actual motion can be produced by translating either the sample or the stylus and cantilever. This motion can be produced by any scanning mechanism, but is typically produced by a piezoelectric translator. All generic scanning mechanisms will be referred to as scanners.
Surface features on the sample interact with the stylus and cause the cantilever to deflect. By measuring the deflection of the cantilever as a function of position over the surface, a map of the surface can be created. In practice, it is often necessary to minimize the force that the cantilever applies to the surface. For this reason, AFMs are usually run under feedback in the "constant force mode." In this mode, the cantilever deflection is kept constant during imaging by moving either the cantilever or the sample with respect to each other. The sample surface is then mapped out by reading a signal indicating the relative motion of the cantilever or sample needed to keep the cantilever deflection constant. When a feedback system is used to keep the cantilever deflection constant, this deflection (and hence force on the sample surface) can be held at a minimum value.
There are also a variety of AC modes where the cantilever is vibrated and features of the sample are sensed by measuring the amplitude, phase, or frequency of the vibrating cantilever. See for example copending application Ser. No. 07/926,175 entitled An Ultra Low Force Atomic Force Microscope by Elings and Gorley.
The first atomic force microscopes used the principal of electron tunneling to detect minute deflections of the cantilever. More recently, two groups (Meyer and Amer, Appl. Phys. Lett., 53 (24), Dec. 12, 1988 and Alexander et al, Appl. Phys. Lett., 65 (1), 1 Jan. 1989 independently succeeded in using the "optical lever technique" to measure cantilever deflections with subnanometer resolution. The "optical lever" technique works in the following way.
Referring to FIG. 1, a prior art AFM system is illustrated. An AFM cantilever 14 is made so that it is sufficiently reflective that it can act as at least a partial mirror. A laser beam 17 from a laser 10 is focused onto one side of the AFM cantilever 14 having a stylus 15 mounted thereon. Laser beam 14 passes through or alongside a scanner 12 using lens 11 and the reflected beam 18 is directed to a position sensitive detector 16, usually a multi-segment photodiode. Cantilever 14 is attached to a mounting substrate 20, which is attached to scanner 12, or to an optional mounting element 19. A cantilever deflection changes the angle of the cantilever with respect to the incoming laser beam 17 and thus moves the reflected laser beam 18 on the position sensitive detector 16 as the cantilever 14 is scanned over the sample 13.
Prior art scanned-stylus AFMs as shown in FIG. 1 focus a laser spot onto one side of a cantilever and then the scanner moves the cantilever over the sample. Since the cantilevers are typically a few hundred microns long and a few tens of microns wide, the focused laser spot must be only 10-30 microns in diameter so that laser light does not spill around the cantilever and onto the sample. If laser light spills around the cantilever, the AFM's sensitivity and therefore vertical resolution is decreased. Also the position sensing photodiode may be disturbed by optical interference from the sample. This means that if a prior art scanned-stylus AFM scans the cantilever more than 10-30 microns, the cantilever will move out from under a stationary laser beam, and the AFM's performance will deteriorate. There is great interest in scanned-stylus AFMs that can scan in the range of 100 microns, much larger than the range of this prior art AFM.
The majority of AFMs that have been built scan a small sample under a fixed stylus and cantilever. There is, however, great interest in AFMs that scan the stylus over a fixed sample. This method has a number of advantages, including the ability to image samples that are too large to be scanned easily. A number of scanned-stylus AFMs have been built and described in the literature. For example, such instruments have been built by G. Meyer and N.M. Amer, Appl. Phys. Lett., 56, p. 2100 (1990), C. B. Prater et al., J. Vac. Sci. Technol., B9, p. 989 (1991), Hipp et al, Ultramicroscopy 42-44, p. 1498 (1992), Putman et al, presented at the OE/LASE '93 Conference, Jan. 19, 1993, Los Angeles, Calif., Baselt and Baldeschweiler Rev. Sci. Instrum. 64, p. 908 (1993), Clark and Baldeschweiler Rev. Sci. Instrum. 64, p. 904, (1993), and by Digital Instruments, U.S. Pat. No. 5,025,658 (E.g., Stand Alone.TM. AFM, Large Sample Scanning Probe Microscope) assigned to the assignee of the present application.
All of the previous instruments suffer from compromises that do not allow them to take full advantage of the capabilities of AFMs that scan by moving the samples instead of the stylus. Prior art scanned-stylus AFMs that use a fixed laser to measure cantilever deflection have a maximum scan size set by the diameter of the laser beam at the cantilever. If the cantilever is scanned a distance larger than the beam size, it will move out from under the beam, and it will no longer be possible to detect the cantilever motion.
A small number of optical lever scanned-stylus microscopes have been built by using a laser beam that is defocused so that in the plane of the cantilever, it is larger than the desired scan range of the cantilever. Microscopes of this type have been built separately by Meyer et al, supra, C. B. Prater et al. supra, Hipp et al, supra, Baselt et all supra, and Clark et al, supra and are typically of the type shown in FIG. 1. The performance of this type of scanned-stylus AFM is greatly diminished at scan ranges of larger than 10-20 microns. This performance loss is seen in the form of images that appear "warped" and uncontrolled force variation across the scan field. These effects have been recently described by Baselt et al, supra.
The reason for this performance loss is as follows. Scanned-stylus AFMs often use a piezoelectric tube translator (12 in FIG. 1) to scan the cantilever over the sample. Typically, one end of the scanner is held fixed and the other end performs the scanning pattern with a pendulum-like motion. This means that scanning the cantilever over the sample surface changes both the angle and position of the cantilever with respect to the incoming laser beam and the angle and position of the cantilever with respect to the position sensitive detector. Some of these problems are described in the paper by Hipp et al, supra. In addition, some commercial cantilevers are not flat, but are instead warped by a small amount. Even in the case that the cantilever motion is small enough that the laser stays on the cantilever, the deflection of the reflected beam can change by large angles as the laser moves on this warped surface. For example Baselt et al, supra, measured false cantilever deflections of 45 nm while scanning a warped cantilever only 750 nm under a fixed laser.
The net result of these angular and position changes is to move the reflected laser beam across the position sensitive detector. In "constant force mode" operation, however, the position of the laser beam on the detector is held constant with a feedback loop that moves the cantilever with respect to the sample. So changes in the relative angles and positions of the laser, cantilever, and detector will cause the feedback loop to exert more or less force on the surface as the cantilever is scanned to keep the position of the laser constant on the detector. These force variations can be very destructive. Many biological samples cannot be imaged with forces larger than one nanoNewton (nN). See for example Hoh and Hansma, Trends in Cell Biology, 2, p. 208 (1992). In the above case, where a warped cantilever caused a false deflection of 45 nm over a 750 nm scan, the force variation could be as high as 1-10 nN for typical cantilevers. Of course, the force variation increases further for larger scan sizes.
An alternative to the scanned-stylus AFMs described above is described by Elings et al (U.S. Pat. No. 5,025,658). This design, shown in FIG. 2, places the AFM cantilever 14 extremely close to the emitting surface of a laser diode 21. The laser light reflects off the cantilever, back into the laser itself. The reflections will cause optical interference that is detected at a photodiode 22 that is contained within the laser package. Laser diode 21 are attached to scanner 12 using a laser mounting member 23. A miniature optical interferometer is formed such that as cantilever 14 deflects the amount of light detected at photodiode 22 changes. Since the laser, photodetector, and the cantilever are closely coupled, the laser and detector always tracks the cantilever, independent of scan size.
This prior art system has a number of disadvantages, however. First, this type of optical interferometer only has sufficient sensitivity for AFM measurements if the laser and cantilever are very closely coupled, typically with less than 100 micron separation. This requires extremely precise alignment of the cantilever with respect to the laser. Second, the piezoelectric translator must carry the weight of a laser diode near the free end of the translator. This can cause a reduction in the mechanical resonant frequency of the translator, making the translator more susceptible to vibrations. Also, because the laser is so close to the cantilever, it is not possible to view the cantilever and sample simultaneously with an optical microscope.
In addition, the optical interferometer used in the FIG. 2 AFM produces an output that changes periodically with distance. The position of cantilever 14 with respect to laser 21 is selected such that a deflection of cantilever 14 causes a change in the interferometer output. For example, the position could be selected so that positive cantilever deflections cause positive increases in the interference output. Then the force feedback will cause cantilever 14 to retract when the interference output increases. But because the interferometer output is periodic, sudden displacements of cantilever 14 can move it to a point where further deflections cause the interference signal to be opposite to that expected, i.e., decrease rather than increase. This sort of error will cause the feedback system to immediately jump the next period ("mode hop") of the interference signal. This makes accurate force control of the prior art scanned-stylus AFM of FIG. 2 very difficult.
Other versions of scanned-stylus AFMs have been built by Putman et al, supra, and commercial instruments have been produced by Topometrix, Inc. These designs are optical lever AFMs where the cantilever is scanned over a fixed sample. Both of these designs mount the laser and cantilever on the same scanning unit, so that they move together. This prior art is shown schematically in FIG. 3. In FIG. 3 a focusing lens 30 is disposed between laser 10 and cantilever 14. This system has the disadvantage that scanner 12 has to carry the weight of laser 10, electronic leads to laser 10, focusing lens 30, and any mechanism for fine tuning the laser position on cantilever 14. All of these can reduce the mechanical resonant frequency of scanner 12 and transmit vibrations to cantilever 14. Also, in the case of a tube scanner, attachment of a laser to the interior of a scanner can make its exchange difficult in case of laser failure.
In addition, these designs have the laser beam 17, cantilever 14 and stylus 15 move with respect to a fixed position sensitive detector 16. So when cantilever 14 is scanned over the surface of sample 13, reflected laser beam 18 will move with respect to fixed position sensitive detector 16, even in the absence of any actual deflection of cantilever 14.
Other versions of scanned stylus AFMs have relied on scanned optical fibers used as interferometers or as a moving light source for the optical lever technique.
Another form of scanned-stylus AFMs use cantilevers that are instrumented with a piezoelectric or a strain gauge (copending application Ser. No. 08/009,076, U.S. Pat. No. 5,266,801 and Tortonese et al, Transducers '91, 1991 International Conference on Solid-State Sensors and Actuators, San Francisco, Calif., 24-27 Jun. 1991, and Tortenese et al, Appl. Phys. Lett., 62, p. 834, (1993), and produce a voltage or a change in resistance that depends on the deflection of the cantilever.
A number of the prior art AFMS, especially those relying on optical interferometers to detect cantilever deflection, closely couple the cantilever and the laser light source and/or the assembly for detecting the cantilever deflection. In some cases this separation is less than 100 microns. This close coupling makes it extremely difficult to also view the cantilever and the area of the sample that it is scanning with an optical microscope.